1. Technical Field
The present invention relates to an ink for inkjet recording, and an ink cartridge and a recorded matter using the ink.
2. Description of the Related Art
Recently, inkjet recording methods have been popular as image forming methods because of having advantages of having simpler process and easier full-colorization than the other recording methods, and producing high-resolution images even with an apparatus having simple composition. The inkjet recording methods have a small amount of ink soar and adhere to recording media such as papers to form images thereon with an inkjet recorder, and applications thereof are expanding, e.g., personal and industrial printers and printings.
In the inkjet recorder, an aqueous ink using a hydrosoluble dye is mostly used as a colorant. However, the ink has disadvantages of having poor weatherability and water resistance. Therefore, a pigment ink using a pigment instead of the hydrosoluble dye has been studied recently. However, the pigment ink is still inferior to the dye ink in colorability, ink discharge stability and preservation stability. In company with improvement of higher-quality image technology of OA printers, even when recorded on plain papers as recording media with the pigment ink, image density equivalent to that of the dye ink is required. However, the pigment ink penetrates into a plain paper as a recording medium and pigment density at the surface of the paper lowers, resulting in lower image density. In order to dry the ink adhering to the recording medium quicker to print quicker, a penetrant is added to the ink for water to penetrate into the recording medium. Then, not only water but also pigment penetrates deeper into the recording medium, resulting in lower image density.
Various methods are disclosed to improve image density.
Japanese Patent No. JP-5001291-B2 (PCT Japanese published national phase application No. 2009-513802) discloses an inkjet composition including a liquid vehicle, a colorant, and a polymer having at least one functional group having a specific calcium index value. Monomers forming the polymer include 4-methacrylamide-1-hydroxybutane-1,1-diphosphonic acid. The colorant is destabilized by the diphosphonic acid group and Ca salt in a paper when contacting the paper, and the resultant printed image improves in quality.
Japanese published unexamined application No. JP-2011-122072-A discloses an ink used for recording on a paper including a hydrosoluble multivalent metal salt. The ink includes (a) a pigment and (b) at least one compound having no surface activating ability, a molecular weight of from 150 to 10000, and a content rate of phosphorous ((p/molecular weight)×100) from a functional group selected from a functional group having a basic skeleton of phosphoric acid and functional group having a basic skeleton of phosphonic acid not less than 1.4. Further, the ink includes the (b) compound of from 1.5 to 10.0% by weight.
Japanese published unexamined application No. JP-2013-103954-A discloses adding a salt having a specified concentration to a self-dispersion type pigment ink to improve intermittent discharge stability, and image density and uniformity of the printed matter due to interaction between a functional group of the self-dispersion type pigment and an electrolyte (the salt).
Japanese Patent No. JP-5001291-B2 does not use a polymer as a dispersant and an additive, but covers a colorant with the polymer. A functional group of the polymer has a diphosphonic acid group adjacent to a hydroxyl group, and the ink has low dispersion stability probably because diphosphonic acid group and the hydroxyl group tend to be bonded by hydrogen bond. 4-methacrlyamide-1-hydroxy-butane-1,1-diphosphonic acid is disclosed as a monomer forming the polymer. However, neither a monomer having the following formula (3) used in the present invention nor a copolymer using the following formulae (3) and (4) is disclosed, and an effect of improving dispersion stability of a pigment is not disclosed, either.
wherein R4 represents a hydrogen atom or a methyl group.
wherein R5 represents a hydrogen atom or a methyl group; R6 represents an alkyl group having 1 to 4 carbon atoms; and each of n and m represents 0 or an integer of from 1 to 30, provided n+m is from 2 to 30.
Japanese published unexamined application No. JP-2011-122072-A discloses none of a copolymer using the formulae (3), effect of image density improvement, and reduction effect of strike-through and dispersion stability of a pigment of a plain paper having a low content of a hydrosoluble multivalent metal salt.
Japanese published unexamined application No. JP-2013-103954-A is assumed to lower the preservability of an ink because of adding salt to the ink to destabilize dispersion sate of a dispersion.